In current and future wireless communication systems, such as the IEEE 802.16 communication standard, known commercially as “WiMAX” and the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) standard, high data rate transmissions for mobile users is a requirement.
High data rate transmissions between two devices when one device on the link is moving create significant challenges. Movement of a device on a wireless link causes variation in the wireless channel with respect to another device on the link. At one instant, when the device is at one position, the channel quality may be good and at another instant when the device is at another position, the channel quality may be poor.
The condition or quality of the channel affects the type of transmission data rate, modulation scheme and transmission scheduling that can be supported. To support high data rate transmissions for mobile devices, a base transceiver station dynamically selects the optimal transmission scheme, such as parameters for one or more of a modulation, data rate and coding level, based on the channel conditions. The transmission schemes may also include different multiple-input multiple-output (MIMO) transmission techniques (when both devices on the link have multiple antennas and other hardware to support MIMO techniques), such as space-time coding, beamforming and spatial multiplexing. When the channel conditions are good, a higher transmission data rate scheme is employed to maximize the transmission throughput and/or improve the quality of service. When the channel conditions are poor, a lower data rate scheme is utilized to guarantee the successful reception of a transmission.
Furthermore, the variation of the wireless channel makes adaptive transmission scheduling challenging. In most adaptive transmission scheduling techniques, channel state or quality information is needed. A first device on the link, e.g., a base transceiver station, estimates the channel state information based on received signals or feedback of specifically configured signals from the other device, e.g., a mobile station, on the link. When the mobile station is moving, there is aging or delay between the time of a scheduled transmission and the time when the channel information was obtained. The channel information obtained at the current frame may not be reliable for the data transmission several frames later when a mobile station is moving since the channel will vary over time with movement of the mobile station. The reliability of the channel information for adaptive transmission schemes depends on the channel variation and the aging from the time when the channel information is obtained to the time of data transmission. Therefore, to effectively schedule high data rate transmissions, the base transceiver station needs to know the channel variation, that is, how slow or fast the channel is varying, i.e., a measure of variation of the channel, as well as an accurate estimate of the channel conditions.
There are numerous channel variation detection heretofore known. One technique is to use so-called pilot signals, which are signals of predetermined known values, from which a receiving device can derive information about variations in the channel. Another technique is to measure the received signal strength indication (RSSI) from received signals and track changes in the RSSI. Still another technique is to have a device transmit the same data in two or more transmission bursts, such as in several different orthogonal frequency division multiple access (OFDMA) symbols, and then compare the difference between the received two or more transmission bursts of the same data. Still another technique is to have one device on the link detect the channel variation and then to transmit a feedback signal to the other device, where the feedback signal contains information describing the measured channel variation.
These channel variation detection techniques have disadvantages. The pilot signal technique requires a sufficient number of pilot signals in order to make accurate estimates of the channel variation. In some systems, there may not be sufficient bandwidth available to have enough pilot signals for channel variation detection. For example, in a WiMAX system, and there are only a few pilot subcarriers in a small uplink transmission burst, and they may be allocated across different sub-carriers. Using RSSI for the channel variation detection requires a relatively long tracking history to be stored by the base transceiver station, and each mobile station would need to transmit with the same power for the RSSI technique to work. However, transmission power adjustment/control is usually required in adaptive transmission schemes. Therefore, RSSI-based channel variation detection has some limitations for practical applications. In many wireless communication systems, such as OFDMA systems, transmitting the same data in two or more different OFDM symbols is difficult to fulfill. Finally, the feedback signal technique requires additional channel bandwidth, and in many wireless communication systems, there is limited or no additional bandwidth available.